C10 Electrical Circuits

Question 1

Kirchhoff’s Second Law

Answer 1

• In any circuit, the sum of the electromotive forces is equal to the sum of the p.d.s around a closed loop.
• Σε = ΣV around a closed loop

Question 2

Series Circuit Laws

Answer 2

• Current is same everywhere (only one pathway for charge to flow).
• e.m.f is shared between components, as there is only one closed loop.

Question 3

In a series circuit which out of two components takes more emf? State why using Ohm’s Law

Answer 3

• The one with the larger resistance.
• V = IR, current is constant, so larger R = larger V.

Question 4

Parallel Circuit Laws

Answer 4

• Current is shared between branches (multiple paths for charges).
• The greater the resistance of the branch, the lower the current.
• P.d. across each branch = e.m.f from power source.

Question 5

Effect of adding resistors in a series circuit

Answer 5

• The length of the path taken by charges increases, so adding more resistors increasing resistance.
• R = R1 + R2 + R3

Question 6

Effect of adding resistors in a parallel circuit, and formula for R total

Answer 6

• The total resistance drops, as each new resistor provides an additional path for the current to flow through the circuit, reducing the overall resistance.
• 1/Rt = 1/R1 + 1/R2…

Question 7

Equations to tackle a complex circuit problem

Answer 7

• Kirchhoff’s Laws
  I = Q/T
  V = W/Q
  P = IV
  V = IR

Question 8

Why do components have an internal resistance?

Answer 8

• Imperfections and impurities in a material can impede the flow of charge.
• Some energy is lost during conversions (e.g. electrical to heat/light/kinetic), and not all energy is available for the circuit.

Question 9

Relationship between Kirchhoff’s Second Law, and e.m.f, and terminal p.d

Answer 9

electromotive force = terminal p.d. + lost volts

Question 10

How does current affect the emf of a cell?

Answer 10

• Increasing current increases lost volts (decreasing terminal p.d).
• As current increases, more charges travel through the cell each second, so more work is done by the charges, increasing lost volts.

Question 11

How do you approximate the e.m.f of a cell in a circuit?

Answer 11

• Use a voltmeter with a high resistance connected directly across a cell.
• This will approximate the e.m.f, as the high resistance will substantially lower the current to where it does not impede the terminal voltage.

Question 12

How do you calculate lost voltage, thereafter terminal p.d. of a component? State formula for calculting emf with reistsance and resistivity.

Answer 12

• Lost voltage = I x Internal Resistance (r)
• Terminal P.D = EMF - Lost Volts
• EMF = I (R + r)

Question 13

Investigating Internal Resistance

Answer 13

• Simple circuit with cell (internal resistance represented by fixed resistor), voltmeter, ammeter, and variable resistor (which will vary current across the cell).
• Graph I against V, y intercept = EMF, gradient = -r

Question 14

Calculating the ratio of voltage between resistors in series

Answer 14

R1 / R2 = V1 / V2

Question 15

What do potential divider circuits do, and when are they used?

Answer 15

• They divide the p.d. of a power supply to any value required.
• Useful for when limiting the voltage e.g. speaker, or when a specific voltage is needed in the moment.

Question 16

Potential Divider Equation

Answer 16

V(out) = (R2 / Rtotal) x V(in)

Question 17

How do you vary V(out) using a potential divider circuit?

Answer 17

• Replace one of the fixed resistors with a variable resistor, e.g. LDR.
• Increasing the resistance of the variable resistor will increase V(out), vice versa.

Question 18

Potentiometer (how they work + advantage)

Answer 18

• A variable resistor with three terminals and sliding contact.
• Adjusting position of contact adjusts p.d between two terminals.
• Very compact, useful for portability.

Question 19

Explain why p.d is the same across all branches in a parallel circuit

Answer 19

• Each branch in a parallel circuit forms its own loop with the power source.
• Kirchhoff’s Second Law tells us the sum of emf must equal sum of p.d around each loop.
• Total p.d across each branch must equal emf